There is an increasing interest in the use of carbon nanotubes in the electronic field, and in general, in information technologies. This interest is particularly due to the electrical properties of carbon nanotubes, which substantially depend on their structure and geometry.
Carbon nanotubes substantially include cylindrical structures of atoms of carbon arranged in an hexagonal configuration, and having a high length-diameter ratio (diameters on the order of several atoms and lengths up to several microns). In particular, nanotubes can be imagined as being substantially formed by a graphite sheet (graphene) rolled up and ending with a half-ball, or crown, of fullerene. Nanotubes can be both single-wall (SWNT, Single-Wall carbon Nano Tube), and multi-wall (MWNT, Multi-Wall carbon Nano Tube) formed by two or more coaxial structures of SWNT. Carbon nanotubes are formed through Chemical Vapor Deposition (CVD), laser ablation, or arc discharge.
On the basis of the diameter and the chirality, i.e., the value of the angle the hexagonal structure of the single graphite sheet is rolled up with, which composes them, carbon nanotubes formed by the graphite sheet with the above techniques can show a metallic or semi-conductive behaviour.
In particular, the natural use of metallic nanotubes is the fabrication of nanowires, which essentially conducts current on a surface and provide small, low resistivity interconnections that conduct high density current. As semiconductors, they can instead be integrating part of a transistor.
To selectively exploit the electrical properties of the typology of carbon nanotubes, several technologies have been devised which enable separation of the semiconductor nanotubes from the metallic ones after they have been developed. The separation of the nanotubes having different chirality, and thus different electrical properties, has been considered up to now as the “holy grail” of the nanotubes field.
In particular, the possibility of controlling the chirality stands for the possibility of making a real new generation of electronic devices comprising circuits only formed by SWNTs, or alternatively by MWNTS, wherein semiconductor nanotubes play the role of active elements (transistors with FETs field effect, logic elements), while metallic nanotubes play the role of connectors.
Some separation technologies are known from the following publications: Separation of Metallic from Semiconducting Single-Walled Carbon Nanotubes, Krupke et al., Science 301, 344-347 (2003), A Route for Bulk Separation of Semiconducting from Metallic Single-Wall Carbon Nanotubes, Chattophadhyay et al., Am. Chem. Soc. 125, 3370-3375 (2003); DNA-Assisted Dispersion and Separation of Carbon Nanotubes, Zheng et al., Nature Mater. 2, 338-342 (2003); Chen et al., A.G. Abstract B26.013, March 2003 Meeting of the American Physical Society; http://www.aps.org/meet/MAR03/baps/abs/S660.html; Structure-Assigned Optical Spectra of Single-Walled Carbon Nanotubes, Bachilo et al., Science 298, 2361-2366 (2002).
Among the above-proposed techniques, the dielectrophoresis seems to be, at the moment, the most promising. In accordance with this technique, a mixture of metallic and semi-conductive carbon nanotubes, just manufactured, can be separated in a solution of micelles of carbon nanotubes in SDS (sodium dodecyl sulfate) using an alternated electrical field at a high frequency (ν>1 MHz).
After the separation of the nanotubes, the real electronic circuits are obtained by a selective assembly of the nanotubes. To achieve this purpose, among the known solutions, the use of an assembly method exploiting the electrical field has been suggested, for example, by contacting single bundles of carbon nanotubes with alternating electrical fields, Krupke et al., Appl. Phys., A 76 397-400 (2003); Self-Assembled, Deterministic Carbon Nanotube Wiring Networks, Diehl et al., Angewandte Chemie International, Edition 41, 353-356 (2002).
This method, although being relatively efficient, is still particularly disadvantageous from an economic point of view. To overcome this drawback, the use of the ULSI technique has been proposed, i.e., a relatively economic process which enables grouping of more than a million electronic components on a single chip.
However, to exploit the advantages of a ULSI process (ultra large scale integration), it is necessary to devise a technique for growing nanotubes which can be integrated directly in a ULSI process. This is done to obtain, directly when forming the nanotubes, several selected circuits at a relatively reduced cost.
In this respect, the above CVD process (chemical vapor deposition) has been indicated, among the several known techniques for developing SWNT or MWNT nanotubes, as the most suitable to be used in a ULSI process (see Growth Mechanisms in Chemical Vapor Deposited Carbon Nanotubes, Vinciguerra et al., Nanotechnology 14(6): 655-660 (2003) and the relative reference therein.
The CVD process also has the advantage of enabling, during the growth, a good control on the position of the carbon nanotubes. However, the CVD process, although being suitable for being integrated in a ULSI process, still has an unsolved drawback.
The drawback stays in that a CVD process does not enable a controlled chirality growth of the carbon nanotubes to be obtained. In particular, a CVD process, which substantially provides the formation of the nanotubes by way of a decomposition of an organo-metal in the gaseous phase with a metallic catalyst present, enables one to obtain nanotubes having a wide distribution of the diameters and of chirality or elicity. This prevents control of the chirality, and thus of the electronic properties of the nanotubes obtained.
As a consequence, in the absence of a control on the chirality of the nanotubes, it is not possible to integrate the growth of the nanotubes in a process directly aimed at the fabrication of electronic circuits.
A method for the formation of nanotubes with controlled chirality is known, for example, from U.S. Pat. No. 6,645,455. The method provides the preparation of carbon nanotubes derived with nucleophilic substituents, as fluorides, which facilitate the solvation and the recovery of the single carbon nanotubes from a solution. This enables the properties of the carbon nanotubes to be controlled. The carbon fluoro-nanotubes obtained are selected on the basis of the desired chirality, and they are bound with precursors of metallic catalysts able to release suitable amounts of reaction catalyst. Once bound to these precursors, the carbon fluoro-nanotubes are used as growth nuclei for obtaining fibers of nanotubes having predetermined chirality.
The known method, although enabling to control the chirality, requires several working steps. There is also a disadvantage of requiring the use of large amounts of compounds, such as the nucleophilic reactants, and the precursors of the metallic catalysts.